1. Field of the Invention
This invention is in the field of absorption spectroscopy, more particularly, it pertains to optoacoustic absorption spectroscopy of condensed matter in the form of thin layers.
2. Description of the Prior Art
Conventional optical spectroscopy techniques tend to fall into two major categories, namely, one can study either the optical photons that are transmitted through the material under study, or those that are scattered or reflected from that material. During the past few years, another optical technique has been developed. This technique, called optoacoustic (OA) spectroscopy, is distinguished from the conventional techniques chiefly by the fact that, even though the incident energy is in the form of optical photons, the interaction of these photons with the material under investigation is studied not through subsequent detection and analysis of photons, but rather through measurement of the energy absorbed by the material due to its interaction with the incident beam. One of the principal advantages of OA spectroscopy is that in principle it enables one to obtain spectra, similar to optical absorption spectra, of any type of solid, liquid, or gaseous material. A recent review of the field of OA spectroscopy can be found in the article by Allan Rosencwaig, "Photoacoustic Spectroscopy" in Advances in Electronics and Electron Physics, Volumne 46, Academic Press (1978).
The presently used method for OA spectroscopy of thin films is an extension of a method for OA spectroscopy of gases. That latter technique consists of confining the gaseous sample to be measured in an experimental chamber having typically two collinearly arranged parallel optical windows having extremely low absorption at the frequency of the probe radiation, irradiating the sample through these windows with chopped monochromatic light, and measuring the chopping frequency change in gas pressure due to the heating effect of the energy absorbed by the gas sample. The pressure variations are generally measured with a sensitive gas microphone. By filling the experimental chamber with a gas transparent to the probe radiation, and by placing a thin film of the condensed matter to be studied into the radiation path inside the experimental chamber, one can determine the absorption spectrum of the thin film material, since the radiant energy absorbed by the film will increase the temperature of the film, resulting in an increase in the gas pressure inside the experimental chamber in proportion to the energy absorbed. Such apparatus has, for instance, been described in an article by E. L. Kerr, Applied Optics, Volume 12, page 2520, (1973). See also U.S. Pat. No. 3,811,782, "Method and Apparatus for Measuring Thin Film Absorption at Laser Wavelengths", E. L. Kerr, May 21, 1974.
The described gas-phase microphone technique for condensed samples has several disadvantages that limit its usefulness. First of all, it relies on the inefficient diffusion of thermal energy from the sample into the gas volume. This makes this technique slow and of low sensitivity, typically useful only if fractional absorption in the film exceeds about 1 percent. Furthermore, effects due to scattered light are troublesome, since such light, when absorbed by the chamber walls, contributes to the pressure increase in the gas due to the resultant heating of the experimental chamber.
A different technique exists for the measurement of optical absorption in bulk samples of condensed matter. In the case of a solid sample a strain transducer, typically a piezoelectric transducer, is placed in direct contact with the sample. A pencil of chopped light, transmitted through the sample, will cause the illuminated volume to expand in direct proportion to the absorption coefficient and the coefficient of thermal expansion of the material. This results in strain waves radiating out from the cylindrical "source" region, and these ultrasonic strain waves can be detected by an appropriately placed transducer. See, for instance, A. Hordvik and H. Schlossberg, Applied Optics, Volumne 16, pages 101-107, (1977), where experimental details of the method are given.
OA spectroscopy on bulk liquid samples can be done by using an experimental cell similar in principle to that described for use with gaseous samples, i.e., having collinear parallel transparent entrance and exit windows. Two types of cells have been used. In one the cell body consists essentially of a piezoelectric transducer in the shape of a cylindrical shell, and in the other a piezoelectric transducer is mounted in an appropriate position in a wall of the experimental cell. Typically in both cases thus the transducer is in direct contact with the liquid sample. For an example of the former approach, see, for instance, the article by A. Rosencwaig, cited above, and an example of the latter can be found in an article by C. K. N. Patel and A. C. Tam in Applied Physics Letters, Volume 34, pages 467-470, (1979). In both types of cell the piezoelectric transducer detects the elastic waves that are generated in the sample liquid by a mechanism analogous to the one described for bulk solid samples, and thus OA spectroscopy on bulk liquid samples is quite similar to such spectroscopy on bulk solid samples.
Prior art bulk techniques that use chopped probe radiation allow the measurement of absorption coefficients in the 10.sup.-4 to 10.sup.-5 cm.sup.-1 range, using light beams of a few hundred mW power. The sensitivity limit of the method is typically determined by thermal interaction of the transducer with scattered radiation. The use of chopped light typically also leads to problems due to heat diffusion because of the relatively long pulse length of chopped light, which makes the prior art techniques essentially optothermal rather than optoacoustic. Furthermore, since typically measurement cells for bulk liquid samples are not disposable, the possibility of unwanted contamination of the sample exists in prior art techniques.